In order to improve security, it has been recently suggested that, in a vehicle, an electronic key be used in which, for example, a transmitting circuit is integrated with a mechanical key. In this structure, an identification code is transmitted from an electronic key in a state of being inserted in an ignition switch, and when the identification code transmitted from the electronic key coincides with a previously set identification code, the electronic key is judged to be true and, based on this, an engine is caused to start up in accordance with a turning operation of the electronic key.
However, in the above-described conventional structure, the identification code itself is transmitted from the electronic key, and therefore, it is possible to read the identification code stored in the electronic key.